Field of the Invention
The invention relates to a method for creating magnetic resonance (MR) images of an object under examination and a magnetic resonance apparatus for implementing such a method. The invention relates, in particular, to methods and MR apparatuses that enable imaging of flow rates.
Description of the Prior Art
The spatially and temporally resolved imaging of flow rates has numerous applications, such as examining the aorta, the carotids, and/or the heart, for example. A multi-dimensional phase contrast MR imaging can be used for measuring flow rates. For a time-resolved determination of the speed, numerous raw data sets must be recorded (acquired) in each examination. In addition to an acquisition with flow compensation, at least one further acquisition occurs, in which a speed encoding gradient or numerous speed encoding gradients are switched on and off, such that, for example, speeds along a specific spatial direction are acquired. The large amount of data that must be acquired (two to three spatial directions, speed encoding in one or more spatial directions, and optionally a temporal resolution) leads to long data acquisition times.
In order to reduce the data acquisition time, methods with sub-scanning have been developed into valuable and promising procedures. Methods of this type can also be implemented as parallel imaging methods, in which data are acquired with multiple antennas. The use of conventional SENSE (“Sensitivity Encoding”) or GRAPPA (“Generalized Autocalibrating Partially Parallel Acquisition”) methods can result in the speed factors being not particularly large and/or the contrast/noise ratio being strongly degraded in an imaging with speed encoding.
DE 10 2011 081 411.6 describes techniques for an MR imaging, in which the MR images are created with iterative reconstruction methods. Various scanning patterns are combined with one another thereby, in order to improve the quality of the reconstructed MR images, even with stronger sub-scanning.
With technologies for iterative reconstruction, matrix elements of a pixel matrix are determined. The pixel matrix can contain pixel values for numerous MR images thereby, which correspond to various speed encodings and/or various times. Many of the conventional technologies for iterative reconstruction do not exploit the fact that stationary tissues and regions having flowing nuclear spins result in specific characteristics of a pixel matrix of this type. Because the various signal contributions from stationary tissues and regions having flowing nuclear spins cannot be systematically taken into account with conventional technologies for iterative reconstruction, reduces the quality of the spatially resolved speed data that can be determined from the reconstructed MR images is reduced when stronger sub-scanning is implemented. Conversely, the time required for the data acquisition is frequently relatively long if flow data are to be determined with an adequate quality.